This invention relates to an automatic document transportation device for a copier and the like for transporting an original document sheet to its scanning position to have its printed surface optically scanned and to a discharge location after it is scanned. More particularly, the present invention relates to a mechanism in such a device by which a document table at the scanning position can be maintained at a fixed distance from a conveyer belt for transporting document sheets.
An automatic document transportation device is typically provided opposite to a document table (for example, of a transparent glass material) of a copier and serves to automatically transport an original to be copied onto the table and discharge it after it has been processed. In general, many document sheets are stacked on top of a document supply table and they are transported onto the table one sheet at a time in response to a control command from the copier. After each sheet is moved to the scanning position and optically scanned, it is discharged into a discharge tray and the next sheet on the supply table is transported onto the table. An automatic document transportation device of the type described above is generally provided with a conveyer belt disposed opposite to the document table for transporting the document to the designated scanning position on the table. With any automatic document transportation device of this type, it is extremely important to accurately position an incoming document sheet on the document table because the position of the original document sheet affects the quality of the produced copies. Accordingly, it is important to maintain a fixed distance between the document table and the conveyer belt. If this distance ceases to be uniform, the coefficients of friction between the belt and the document sheet and between the document sheet and the document table become different and the document sheet may come to be transported obliquely to the direction of its motion. It then becomes impossible to deliver a document sheet accurately to the scanning position. Prior art methods of maintaining a fixed distance between them included providing spacers to the four corners of the frame of the document transportation device opposite to the main body of the copier.
An automatic document transportation device is usually built with a strong supporting structure as a combination of frames and stays. Various components of the device such as a document transportation unit, a driving unit for driving the document transportation unit and a control unit for controlling the operation of the driving unit are attached to this structure and an outer housing is also attached to this supporting structure to cover these components. In other words, a conventional automatic document transportation device has a strong supporting structure as its main body to which many of its components as well as its outer cover are attached. Since these frames, stays, components and outer housing pieces are mass-produced, errors are inevitable in their measurements and when they are assembled together, a shearing strain is accordingly unavoidable. Since the magnitude of such a strain varies from one completed assembly to another, the common practice has been to adjust the automatic document transportation devices individually as they are attached to the main bodies of copiers. Such adjustments are extremely time-consuming. In particular, the aforementioned distance between the document table and the conveyer belt would not be adjustable if the document transportation unit were strained. Thus, the adjustments are effected only after the strain in the device is eliminated.